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Greenhouses should provide a controlled environment for plant production with sufficient
sunlight, temperature and humidity. Greenhouses need exposure to maximum light, particularly
in the morning hours. Consider the location of existing trees and buildings when choosing your
greenhouse site. Water, fuel and electricity make environmental controls possible that are
essential for favorable results. For this reason, use reliable heating, cooling and ventilation.
Warning devices might be desirable for use in case of power failure or in case of extreme
temperatures.

The house temperature requirements depend upon which plants are to be grown. Most plants
require day temperatures of 70 to 80 degrees F, with night temperatures somewhat lower.
Relative humidity may also require some control, depending on the plants cultured.

Tropical plants usually grow best in high humidity with night temperatures of 70 degrees F.

Heating

Georgia greenhouses must be heated for year-round crop production. A good heating system is
one of the most important steps to successful plant production. Any heating system that provides
uniform temperature control without releasing material harmful to the plants is acceptable.
Suitable energy sources include natural gas, LP gas, fuel oil, wood and electricity. The cost and
availability of these sources will vary somewhat from one area to another. Convenience,
investment and operating costs are all further considerations. Savings in labor could justify a
more expensive heating system with automatic controls.

Greenhouse heater requirements depend upon the amount of heat loss from the structure. Heat
loss from a greenhouse usually occurs by all three modes of heat transfer: conduction, convection
and radiation. Usually many types of heat exchange occur simultaneously. The heat demand for a
greenhouse is normally calculated by combining all three losses as a coefficient in a heat loss
equation.

Conduction

Heat is conducted either through a substance or between objects by direct physical contact. The
rate of conduction between two objects depends on the area, path length, temperature difference
and physical properties of the substance(s) (such as density). Heat transfer by conduction is most
easily reduced by replacing a material that conducts heat rapidly with a poor thermal conductor
(insulator) or by placing an insulator in the heat flow path. An example of this would be
replacing the metal handle of a kitchen pan with a wooden handle or insulating the metal handle
by covering it with wood. Air is a very poor heat conductor and therefore a good heat insulator.

Convection

Convection heat transfer is the physical movement of a warm gas or liquid to a colder location.
Heat losses by convection inside the greenhouse occur through ventilation and infiltration (fans
and air leaks).

Heat transfer by convection includes not only the movement of air but also the movement of
water vapor. When water in the greenhouse evaporates, it absorbs energy. When water vapor
condenses back to a liquid, it releases energy. So when water vapor condenses on the surface of
an object, it releases energy to the outside environment.

Radiation

Radiation heat transfer occurs between two bodies without direct contact or the need for a
medium such as air. Like light, heat radiation follows a straight line and is either reflected,
transmitted or absorbed upon striking an object. Radiant energy must be absorbed to be
converted to heat.

All objects release heat in all directions in the form of radiant energy. The rate of radiation heat
transfer varies with the area of an object, and temperature and surface characteristics of the two
bodies involved.

Radiant heat losses from an object can be reduced by surrounding the object with a highly
reflective, opaque barrier. Such a barrier (1) reflects the radiant energy back to its source, (2)
absorbs very little radiation so it does not heat up and re-radiate energy to outside objects, and (3)
prevents objects from “seeing” each other, a necessary element for radiant energy exchange to
occur.

Factors Affecting Heat Loss

Heat loss by air infiltration depends on the age, condition and type of greenhouse. Older
greenhouses or those in poor condition generally have cracks around doors or holes in covering
material through which large amounts of cold air may enter. Greenhouses covered with large
sheets of glazing materials, large sheets of fiberglass, or a single or double layer of rigid or
flexible plastic have less infiltration (Figure 1).

Figure 1. Energy loss due to infiltration.

The greenhouse ventilation system also has a large effect on infiltration. Inlet and outlet fan
shutters often allow a large air exchange if they do not close tightly due to poor design, dirt,
damage or lack of lubrication. Window vents seal better than inlet shutters, but even they require
maintenance to ensure a tight seal when closed.

Solar radiation enters a greenhouse and is absorbed by plants, soil and greenhouse fixtures. The
warm objects then re-radiate this energy outward. The amount of radiant heat loss depends on the
type of glazing, ambient temperature and amount of cloud cover. Rigid plastic and glass
materials exhibit the “greenhouse effect” because they allow less than 4 percent of the thermal
radiation to pass back through to the outside.

Figure 2. Energy losses and gains in a greenhouse.

Heat Loss Calculations

Heat loss by conduction may be estimated with the following equation:
Q = A (Ti - To)/R

Table 1 lists different materials commonly used in greenhouse construction and their associated
R values. Table 1 also lists overall R values for various construction assemblies. Note that high R
values indicate less heat flow. Building materials that absorb moisture will conduct heat once
they are wet. Use vapor barriers to protect materials that are permeable to water vapor. Heat is
also lost to the ground underneath and beside a greenhouse. The perimeter heat loss may be
added to other losses using Table 1 and the equation:

Q = PL (Ti - To)

P = Perimeter heat loss coefficient, BTU/ft ºF hr

L = Distance around perimeter

Table 1. Heat Flow Through Various Construction Materials and Assemblies.

Table 2 lists estimates of air exchanges through types of greenhouses. The number of air
exchanges per hour will vary depending on the type and condition of the greenhouse and the
amount of wind.

Table 2. Natural Air Exchanges for Greenhouses

Construction System

Air Exchanges per Hour1

New Construction, glass or fiberglass

0.75 to 1

New Construction, double layer plastic film

0.5 to 1.0

Old Construction glass, good maintenance

1 to 2

Old Construction glass, poor condition

2 to 4

1Low wind or protection from wind reduces the air exchange rate.

Minimum Design Temperatures

A good outside temperature to use in heater design calculations (to select heater size) can be
found by subtracting 15 degrees F from the average daily minimum January temperature (see
Table 3). Another requirement the heater must meet is to provide enough heat to prevent plants
from freezing during periods of extremely low temperatures. The minimum temperatures for
various locations within Georgia are also shown in Table 3.

Table 3. Climatic Conditions in Georgia (1948-2004)

Location

Minimum Temperature ºF and (Year
Occurring)

Average Daily
Minimum
January
Temperatures
(ºF)

Atlanta

-8 (1985)

33.6

Athens

-4 (1985)

33.2

Augusta

-1 (1985)

33.6

Columbus

-2 (1985)

36.4

Macon

-6 (1985)

35.8

Rome

-9 (1985)

30.5

Savannah

3 (1985)

39.0

Tifton

0 (1985)

38.0

Valdosta

9 (1981)

38.6

Example:

Maintain a temperature of 65 degrees F inside a double layer plastic greenhouse with dimensions
as shown in Figure 3 with no foundation insulation. Assume an Augusta location.

Surface Area:

Walls

7 x 100 x 2

=

1400.0 ft²

Roof

16.86* x 100 x 2

=

3372.0 ft²

Ends

(32 x 7 + 5.33 x 16)2

=

618.6 ft²

5390.6 ft²

* This dimension can be determined by drawing the greenhouse cross-section to scale and measuring this length along the rafters.

At an Augusta location and an average daily minimum January temperature of 33.6 degrees F,
the design temperature would be about 18.6 degrees F, so use 20 degrees F. This requires a 45-degree F rise above design temperature; and, with double layer plastic, the R-value will be 1.43.

Conduction Heat Loss, QC:

= Area x ΔT/R

= 5391.0 x 45/1.43

= 169,647 BTU/hr

Volume:

= (7 x 32 x 100) + (16 x 5.33 x 100)

= 22,400 + 8,528

= 30,928 ft³

Air Infiltration Losses, QA:

= 0.02 x Volume x C x ΔT

= 0.02 x 30,928 x 1.0 x 45

= 27,835 BTU/hr

Perimeter Heat Loss, QP:

= P x L x (ΔT)

= 0.8 x 264 x 45

= 9,504 BTU/hr

Total Heat Loss, QT:

= QC + QA + QP

= 169,647 + 27,835 + 9,504

Heat Required = 206,986 BTU/hr

The coldest temperature recorded in Augusta is -1 degree F and, with a 45-degree F temperature
rise, the plants should not be in jeopardy from freezing. An increase in heat requirement of
approximately 20 percent would be necessary if the house were located on a windy hill.

Figure 3. Gable double layer polyethylene greenhouse.

Other Heating System Design Considerations

Plastic greenhouses often have a humidity buildup within the enclosure since almost no cracks or
openings exist as in a glass house. High humidity can lead to increased occurrence of leaf and
flower diseases. A forced air heating system helps mix the air within the house and helps prevent
temperature variation within the house. In fact, it is desirable to have fans along the walls to
circulate and mix the warm air with the cooler air near the surface. They can be operated
continuously during cold periods even if the heater is not on.

Duct systems to evenly distribute the heated air from the forced warm air furnace are desirable.
Two or more small heating units are preferable to one larger unit, since two units offer more
protection if one unit malfunctions.

A warning device is good insurance should the heating system malfunction or if a power failure
occurs. Some greenhouse operators prefer to have a battery powered alarm system to warn them
if the temperature gets out of the acceptable range.

Ventilation

Ventilation reduces inside temperature during sunny days and supplies carbon dioxide, which is
vital to the plants’ photosynthesis. Another advantage of ventilation is to remove warm, moist air
and replace it with drier air. High humidity is objectionable since it causes moisture condensation
on cool surfaces and tends to increase the occurrence of diseases.

Some glass houses are ventilated by manually operated ventilators in the roof. This method is
usually not satisfactory for ventilating plastic covered houses due to the rapid temperature
fluctuations possible. Ventilating fans are highly recommended in Georgia.

Winter ventilation should be designed to prevent cold drafts on plants. This has been a problem
with some systems using shutters at one end of a house and an exhaust fan at the other. The
problem can be minimized by placing the intake high in the gable and using baffles to deflect the
incoming air.

Draft-free winter ventilation can be provided by using the convection tube system, consisting of
exhaust fans and fresh air inlets located in the gable and end wall. This is connected to a thin
plastic tube extending the length of the greenhouse. The tube is suspended on a wire near the
ridge and has holes along its entire length. The fans can be thermostatically controlled. Fan
operation produces a slight air pressure drop inside the greenhouse, causing fresh air to flow into
the inlet and inflate the tube, which discharges air into the house through the holes in the tube.
The holes emit “jets” of air that should project horizontally to provide proper distribution and
mixing with warm air before reaching the plants.

The thermostat stops the fans when the desired temperature is reached; the tube collapses and
ventilation stops. In a tightly constructed greenhouse, it makes little difference where fans are
located in convection tube ventilation since the air distribution is determined by the tubes. Less
fan capacity is usually required for the convection tube system than for any other winter
ventilation system. Additional air is necessary as the outdoor temperature rises to the point where
full capacity of the tube is reached. The outside air is usually warm enough by this time to be
admitted through doors or other openings at plant level.

Fans may be added or possibly combined with a cooling pad for use in evaporative cooling. In
fact, air may be pulled through the pad with or without water in the pad. In warm periods, enough
air needs to be pulled from the house to provide a complete air exchange every 60 seconds.
Control fans by a thermostat or humidistat to provide proper temperature and humidity.

Greenhouses equipped with an evaporative cooling pad system having three fans or fewer should
have one fan with a two-speed motor to prevent excessive temperature fluctuations and fan
cycling. Select all fans to operate against a slight pressure (⅛ inch static water pressure). Fans
not rated against slight pressure usually move only 60 to 70 percent of the rated air flow when
installed in greenhouses. It is recommended that only fans that have been tested and their
performance verified by an independent testing lab, such as AMCA, be used, since that is the
only assurance that the design ventilation rate is being achieved.

Exhaust Fans in End Wall

Fans in the end wall (Figure 4) are the most common method of forced ventilation. The air enters
through the motorized shutter (winter) and is pulled through the greenhouse by the exhaust fans.

Figure 4. Fans in end wall.

The exhaust fans should be able to move small air volumes without drafts (winter) and yet
provide enough fan capacity for an air exchange within the house each minute during summer.
One air exchange per minute (without evaporative cooling) should keep the temperature about 8
degrees F higher than outside temperatures. One-half of this air volume will produce about a 15-degree F temperature rise, while two air exchanges per minute will cause a temperature rise of
about 5 degrees F. Ideally, the length of the house should not exceed 125 feet using this method.
Houses up to 250 feet long, however, have been satisfactorily ventilated using this method.
Temperature variations are greater in longer houses, so higher ventilation rates are desirable. No
air must be allowed to enter the house at the sides or at the fan end.

Glazing in glass houses must be well set and the houses in good repair to prevent significant
quantities of air leaking into the house. If cooling pads are used during summer, disconnect the
motorized shutter and close it to prevent hot air from entering through the shutter and bypassing
the cooling pads. You can connect a perforated plastic tube to the same inlet shutter to provide
good air distribution for cold weather ventilation.

The same principle applies for multiple ridge houses, provided each end wall is so equipped. One
two-speed fan is usually used in small hobby houses.

The total inlet opening in the end wall for summer ventilation (shutter and evaporative pad vent)
should provide about 1.5 square feet per 1,000 cubic feet per minute of air moving through the
operating fans. The motorized shutter and one or two fans might be connected on one thermostat
while the remaining fans are connected to a different thermostat, with air being supplied to these
fans through the vent panel containing the evaporative pad.

Pressure Fans in End Walls

Ventilation for greenhouses that are 100 feet or shorter can be accomplished by mounting
pressure fans, which blow air into the house, high in the end walls. See Figure 5.

Figure 5. Pressure fans mounted high in the end walls.

The fans in the end wall are usually two-speed and controlled by separate thermostats. To avoid
high velocity air striking plants, a baffle is placed in front of the fans to direct the air in the
direction desired. The fans should have a protective hood to prevent rain from being blown into
the house.

One pressurized system where evaporative cooling is possible is shown in Figure 6. This system
places the pressure fans in the side wall. The pressurized system with fans in the side wall does
not work well when the foliage is dense and lots of tall, growing plants are present. Notice the air
outlet and inlet are on the same side of the house in this case, with a box enclosure around the fan
where cooling pads are installed.

Figure 6. Pressure fans mounted in the sidewalls.

Evaporative Cooling

The heat absorbed on a dark surface perpendicular to the sun’s rays can be as high as 300
BTU/HR per square foot of surface. So it would be possible, theoretically, for a greenhouse to
absorb 300 BTUs per hour for each square foot of floor area. This excessive energy leads to heat
buildup and, on warm days, can cause plants to wilt.

Excessive heat buildup can often be prevented with shading materials such as roll-up screens of
wood, aluminum or vinyl plastic as well as paint-on materials (shading compounds). Roll-up
screens, which work well in hobby houses, are available with pulleys and rot-resistant nylon
ropes. These screen can be adjusted from outside as temperature varies. Radiation can be reduced
by 50 percent with this method, which should reduce temperature rise proportionally if
ventilation rate remains constant. Shading also reduces light striking the plants, which may limit
their growth rate since light is essential to photosynthesis. This is a trade-off that is sometimes
necessary to reduce temperatures.

If summer temperatures exceed those considered acceptable and cannot be corrected with
reasonable ventilation rates and shading, the only alternative is evaporative cooling. A fan and
pad system using evaporative cooling eliminates excess heat and adds humidity. This reduces
plant moisture losses and, therefore, reduces plant wilting. Temperature is lowered, humidity is
increased and watering needs are reduced.

An evaporative cooling system moves air through a screen or spray of water in such a manner
that evaporation of water occurs. About 1,000 BTUs of heat are required to change 1 pound of
water from liquid to vapor. If the heat for evaporation comes from the air, the air is cooled.
Evaporation is greater when the air entering the system is dry; that is, when the relative humidity
is low, allowing the air to evaporate a lot of water. The water holding ability of air is expressed in
terms of relative humidity. A relative humidity of 50 percent, for example, means the air is
holding one-half of the maximum water that the air could hold if saturated at a given
temperature.

Theoretically air can be cooled evaporatively until it reaches 100 percent relative humidity.
Practically, a good evaporative cooler can reach about 85 percent of this temperature drop. The
cooling effect for 85 percent efficient evaporative coolers is shown in Table 4.

Evaporative coolers are more effective when the humidity is low (Table 4). Fortunately, relative
humidities are usually low during the warmest periods of the day. Solar heat entering the house
offsets some of the cooling effect. A well-designed ventilation system pro-viding one air volume
change per minute is essential for a good evaporative cooling system. A solar heat gain of 8-10
degrees F can be expected using one air change per minute. If the outside air were 90 degrees F
and relative humidity were 70 percent, the resulting temperature within the house would be about
93 degrees F (83 degrees F from Table 4 plus 10 degrees F).

Table 4. Cooling Capacity of 85 Percent Efficient Evaporative Coolers

Outside Air

Relative Humidity

at 30%

at 50%

at 70%

at 90%

Outside Air Temperature ºF

Cooled Air Temperature ºF

100

79

86

91

96

90

70

77

83

87

80

63

69

74

77

70

54

60

64

68

If a cooling efficiency of 85 percent is to be realized, at least 1 square foot of pad area (aspen
fiber) mounted vertically should be provided for each 150 CFM of air circulated by the fans.
Many pad materials have been used successfully, provided a complete water film does not form
and block air movement through the wet pad. Table 5 gives recommended air flow through
various pad type materials.

Table 5. Recommended Airflow Rate through Various Pad Materials.

Pad Type

Airflow Rate
through
Pad (CFM/ft2)

Aspen fiber mounted vertically (2-4 in. thick)

150

Aspen fiber mounted horizontally (2-4 in. thick)

200

Corrugated cellulose (4 in. thick)

250

Corrugated cellulose (6 in. thick)

350

Figure 7. Typical evaporative cooling system.

Aspen pads are usually confined in a welded wire mesh. A pipe with closely spaced holes allows
water to run down a sheet metal spreader onto the pads (Figure 7). The flow rate of the water
supplying header pipe is listed in Table 6. Water than does not evaporate in the air stream is
caught in the gutter and returned to a reservoir for recycling. The reservoir should have the
capacity to hold the water returning from the pad when the system is turned off. Table 6 shows
recommended reservoir capacity for different type pads.

A cover of some sort is needed to prevent air flow through the pads during cold weather. These
can be manually operated or automated. Float control easily controls water supply. It is desirable
to use an algaecide in the circulating water to prevent algae growth on the pads. You must,
therefore, prevent rain water from entering the evaporative cooling water, causing dilution of the
chemical mixture.

Evaporative pads in an endome on the suction side of fans that discharge air into houses
(pressure fans) have not worked well, primarily due to the distribution of the cooled air. The
same is true of package unit evaporative coolers where poor air distribution is concerned. These
units can handle air volumes of 2,000 to 20,000 CFM. The problem with them is the difficulty
providing uniform cooled air distribution. The closer the units are spaced along the walls, the
better the air distribution will be. Package coolers have been used in small houses, and in houses
with good air distribution, with considerable success. The pressurized system forces air, which
must displace air within the house, into the greenhouse. Vents must be provided for air
circulation.

Mist Cooling

Evaporative cooling by spraying tiny water droplets into the greenhouse has met with limited
success. The droplets must be tiny, and this requires tiny, closely spaced nozzles operated at
relatively high pressures — an expensive design. Water must be well filtered to prevent nozzles
from clogging. Uniform distribution of the water droplets throughout the house is difficult to
accomplish.

If the mist system carries any minerals in the water, deposits will be left on plant foliage. This
accumulation can reduce photosynthesis substantially and can lead to salt toxicity. The mist
system can also cause wet foliage, leading to disease problems, particularly when the droplet size
is too large.

Mist cooling does not cool as effectively as a conventional evaporative cooling pad system but it
is less expensive. The system requires no collection pan or sump. It can cause runoff or puddling
beneath the pads if all the water sprayed on the pads is not vaporized.

A system that is actually a combination of a cooling pad and misting (or fogging) system is
shown in Figure 8. This is sometimes called a “fogging pad” system. Some growers have used it
with success.

Figure 8. Mist nozzle used as evaporative cooling.

The system should provide approximately 20 gallons of water per minute to be sprayed on the
pad (typically 20, 1-gpm spray nozzles) for each 48-inch fan in the ventilation system. This
amount of water, however, will not always be needed.

Warmer air will evaporate water faster than cooler air. The amount of water added to the pads can be
adjusted using a combination of valves, time clocks and thermostats. As the
temperature in the greenhouse increases, so does the frequency of mist nozzle operation.

Natural Ventilation

Some greenhouses can be ventilated using side and ridge vents, which run the full length of the
house and can be opened as needed to provide the desired temperature. This method uses thermal
gradients, creating circulation due to warm air rising.

Houses with only side vents depend upon wind pressure for ventilation and are usually not
satisfactory. The warm air must be allowed to rise through the ridge vent while cooler air enters
along the sides. The vent size is important. Ridge vents should be about one-fourth the floor area
and the side vents about the same size. The roof vents should open above the horizontal position
to provide about a 60-degree angle to the roof. Most of these vents are manually operated.

References

AMCA. Air Movement and Control Association International, Inc. 30 West University Dr.,
Arlington Heights, IL 60004-1893.

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